Ammonia volatilization during drought in perennial C₄ grasses of tallgrass prairie

We measured foliar NH₃ volatilization as part of our study of the decrease (up to 40%) in shoot N concentration during drought in three perennial C₄ grasses of tallgrass prairie. Volatilization of recently expanded leaves was quantified using cuvettes and acid traps for Spartina pectinata, Andropogon gerardii, and Schizachyrium scoparium, a mesic, intermediate, and xeric species, respectively. In general, volatilization decreased during drought, approaching zero as stomates closed, and increased with plant N status and drought tolerance. Prior to drought, NH₃ volatilization was greater in xeric than mesic species (179 and 131 vs. 115 ng m^-2 s^-1 for individual leaves of S. scoparium and A. gerardii vs. Sp. pectinata). During a 2-3 week drought, whole-shoot volatile N losses can exceed 5% of total plant N in these species, accounting for 2-10% of the decrease in shoot percent N (again, xeric > mesic). Drought-induced N retranslocation of shoot N to roots and rhizomes is responsible for c. 63% of the decrease in percent N in Sp. pectinata, 28% in A. gerardii, and 8% in S. scoparium. The remainder of the decrease in percent N is attributable to growth dilution of existing shoot N, accounting for 34, 65, and 87% of the decrease in shoot percent N during drought in Sp. pectinata, A. gerardii, and S. scoparium, respectively. Thus, the relative importance of volatilization, retranslocation, and dilution in decreasing foliar percent N during drought in prairie grasses is species dependent and related to drought tolerance.